I. Technical Field
The present invention relates to an operating method of a solid electrolyte fuel cell capable of being operated for a long period of time without degrading the performance thereof, and additionally relates to a solid electrolyte fuel cell capable of being used for a long period of time without degrading the performance thereof.
II. Description of the Related Art
In general, solid electrolyte fuel cells can use, as fuel, hydrogen gas, natural gas, methanol, coal gas or the like, accordingly solid electrolyte fuel cells enable promotion of the adoption of an alternative energy to petroleum in electricity generation to utilize the exhaust heat, and hence have been attracting attention also from the viewpoints of natural resources saving and environmental issues. As shown in FIG. 2 illustrating a sectional view, such a conventional solid electrolyte fuel cell generally has a fundamental structure in which: a power generating cell 4 having a solid electrolyte 1 composed of an oxide, an air electrode 2 laminated on one side of the solid electrolyte 1, and a fuel electrode 3 laminated on the other side of the solid electrolyte is provided; an air electrode current collector 5 is laminated on the outer side of the air electrode 2 of the power generating cell 4; a fuel electrode current collector 6 is laminated on the outer side of the fuel electrode 3 of the power generating cell 4; an air electrode current collector-side separator 7 is laminated on the outer side of the air electrode current collector 5; and a fuel electrode current collector-side separator 8 is laminated on the outer side of the fuel electrode current collector 6. The air supplied to the air electrode current collector 5 is supplied through an air supply passage 11 including a pipe provided by being connected to the air electrode current collector-side separator 7, and the hydrogen supplied to the fuel electrode current collector 6 is supplied through a fuel supply passage 10 including a pipe provided by being connected to the fuel electrode current collector-side separator 8.
Lanthanum gallate oxide ion conductors are known to be used as the solid electrolyte 1 forming the power generating cell 4 of the solid electrolyte fuel cell; the lanthanum gallate oxide ion conductors are known to be the oxide ion conductors represented by a general formula, La1-XSrxGa1-Y-ZMgYAZO3 (wherein A is one or more of Co, Fe, Ni and Cu, X=0.05 to 0.3, Y=0 to 0.29, Z=0.01 to 0.3, and Y+Z=0.025 to 0.3) (see Japanese Patent Laid-Open No. 11-335164).
Additionally, the fuel electrode 3 is known to be formed of a porous sintered body composed of particles of a B-doped ceria (wherein B is one or more of Sm, La, Gd, Y and Ca) represented by a general formula, Ce1-mBmO2 (wherein B is one or more of Sm, La, Gd, Y and Ca, and m satisfies the relation 0<m≦0.4) and nickel particles, and the porous sintered body is also known to have a structure in which the nickel particles are mutually sintered to form a framework structure, and the particles of the B-doped ceria having a particle size of 0.1 to 2 μm form a network structure and attach to the surface of the porous nickel having such a framework structure. Further, the air electrode 2 is formed of a ceramic such as (Sm,Sr)CoO3 or (La,Sr)MnO3 (see Japanese Patent Laid-Open No. 11-297333).
On the other hand, the fuel electrode current collector 6 is generally formed of Ni mesh. Additionally, the air electrode current collector 5 is known to be generally formed of platinum mesh; however, in the past years, the following materials come to be used instead of expensive platinum mesh: inexpensive silver porous materials such as silver mesh, silver felt and silver foam, and silver-coated porous metal materials formed by coating the surface of meshes, felts or metal foams made of metals other than silver with silver (see Japanese Patent Laid-Open No. 2002-280026).
Further, the air electrode current collector-side separator 7 and the fuel electrode current collector-side separator 8 of the solid electrolyte fuel cell are usually formed of a stainless steel, such as SUS 430, excellent in high temperature corrosion resistance; the surface of the air electrode current collector-side separator 7 is especially susceptible to oxidation, hence the surface of the air electrode current collector-side separator 7 is oxidized to increase the contact resistance with the air electrode current collector 5 to significantly waste the electromotive force, and consequently the electricity generation efficiency is significantly decreased; thus, as shown in FIG. 2, a silver plating layer 9 generally comes to be formed on the surface of the air electrode current collector-side separator 7 (see Japanese Patent Laid-Open No. 2002-289215).
A solid electrolyte fuel cell having such a configuration as described above usually operates within a temperature range from 650 to 1000° C.
However, the following problems may arise in the case where a solid electrolyte fuel cell uses an air electrode current collector formed of porous silver materials such as silver mesh, silver felt and silver foam, or an air electrode current collector formed of silver-coated porous metal materials formed by coating the surface of meshes, felts or metal foams made of metals other than silver with silver, and a silver plating layer is formed on the surface of the air electrode current collector-side separator. That is, when such a solid electrolyte fuel cell is operated for a long period of time within a temperature range from 650 to 1000° C., silver is evaporated within this temperature range although the evaporation amount is extremely small. Thus, when such a solid electrolyte fuel cell is used for a long period of time, the silver on the air electrode current collector and the sliver plating layer formed on the surface of the air electrode current collector-side separator are gradually diminished, consequently, the performance of the air electrode current collector is degraded and the contact resistance between the air electrode current collector-side separator 7 and the air electrode current collector 5 is increased due to the diminishing of the sliver plating layer formed on the surface of the air electrode current collector-side separator, and thus the electromotive force is significantly wasted to decrease the electricity generation efficiency.